Observations will be made on Drosophila neurons and muscle cells during their differentiation in vitro. An electron micrograph record will be prepared showing ultrastructural changes during this time. These cells will be challenged by inhibitors of nucleic acid and protein synthesis in order to determine the dependence of differentiation of these events. Mutants will be sought that yield cells incompetent to differentiate in vitro. The mutants will be used to analyze cell differentiation pathways, both genetically and biochemically.